that said, optical performance is comparable to these primes and for many this one lens could replace them for most purposes

similar optical performance to the f/2.8 lenses for APS-C dSLRs and whilst you give up 1 stop shallow depth of field (DOF) capability, the lens is around half the weight which makes for far more portability, plus you get very fast CD-AF and it is almost waterproof, while filters are smaller and thus less expensive.

specs

if you put the ring in the MF position, set your distance, then push it back to AF and focus, it remembers the MF position. This means you can pull focus instantly between any two distances simply by pulling the ring backwards! Better still, the distance is held regardless of the zoom setting.

more than twice the price and 2.5x as heavy at 805g, and a massive, expensive 82mm filter

much slower CD-AF

not as “waterproof”

but you do get 2 stops more shallow depth of field (DOF) - but if you really need this level of DOF control on Micro Four Thirds system, you can resort to fast aperture primes such as a 42mm f/1.2 which will give even more shallow DOF

this is the better lens for wedding photographers and outdoor fashion work but for most other uses, the Olympus lens will suffice, especially when complimented with a fast prime or two.

optically best at 25mm f/2.8, where edges are as sharp as Olympus mZD 25mm f/1.2 PRO lens at f/2.8 with similar vignetting and almost zero distortion, and better CA, but centre is a little sharper on the prime lens

“The Panasonic G X VARIO 12–35 mm f/2.8 ASPH. P.O.I.S set the standards pretty high in this category but the Olympus company managed to meet them without any problem. First of all they produced a bit cheaper lens with a wider focal range. When it comes to the image sharpness the Panasonic is a bit more even but the Olympus provide a tad more lpmm at the maximum relative aperture. Apart from that you get slightly lower distortion and chromatic aberration, noticeably lower vignetting and distinctly better performance against bright light. The Panasonic, for a change, corrects the coma better and its off-focus images look nicer; it is also physically lighter and smaller, despite built-in image stabilization, and it is produced in Japan. ”

strangely, resolution drops wide open at 40mm so really need to shoot f/4 at 40mm.